1. Field of the Invention
This invention relates to redundant power supply circuits for applications in which each of a number of loads is connected by one or another semiconductor switch to one or the other of two redundant supplies in response to external control signals which operate the switches, and more particularly to an improved circuit which provides protection to the loads from power supply overvoltage in MOSFET switched power supply circuits.
2. Description of the Prior Art
In the prior art, overvoltage protection is often provided by a so-called crowbar circuit, which shunts the supply voltage and protects the load from the overvoltage condition. While generally satisfactory for certain applications, the crowbar circuit dissipates power and can create current stress conditions. Alternatively, some power supplies have internal overvoltage protection. However, to meet isolation requirements, power supplies that provide internal overvoltage protection require a link between the overvoltage event on the secondary side of the supply with the disabling power converter control circuitry on the primary side without ohmic connections between the primary and secondary. This is usually accomplished by one of the following methods. One method adds a slave winding to the isolation transformer to provide a representation of the output voltage. However, this representation of the output voltage is not precise. Another method references all of the control circuits to the secondary side. However, this method requires, within the supply, a dedicated bias supply and a transformer isolated signal to drive MOSFET switches, and thus this method requires appreciable physical volume to implement. Yet another method uses optoisolators, but these can degrade in a space environment. In addition, the optoisolator overvoltage protection circuit must be separate from the main loop reference and error amplifier in order to eliminate the potential for a single point failure that could cause an unrecoverable overvoltage condition.